Pausing for just 25 seconds before buying a chocolate bar can be enough to see off the craving in favour of a more healthy snack, new research suggests.

A trial using vending machines in the US found that having to wait for sugary treats made them less desirable.

Scientists installed countdown timers on the machines, forcing customers who had ordered chocolate or other unhealthy products to wait 25 seconds before the items became available.

Having to wait for something makes it less desirable

Dr Brad Appelhans, Rush University Medical Centre

They were able to change their mind and select a healthy snack at any point during the countdown, which would then become instantly available, meanwhile those who selected something healthy in the first instance were not forced to wait at all.

Over 14 months the team found the delay prompted up to a five per cent increase in the purchase of healthy snacks, but that the new software did nothing to harm the revenues brought in by the machines.

British obesity campaigners have welcomed the study and said there will be “no excuse” not to install the technology on all modern vending machines.

Approximately one in nine machines operating in Europe is thought to be located in the UK, with more than 7 billion items dispensed each year, generating a profit of well over £1.5 billion.

Previous interventions to try and reduce the harm caused by the easy availability of sugary snacks have focused on the complete removal of such products, or the machines altogether.

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The Government wants a 20 per cent reduction in the sugar content of sweetsCredit: PA

But many organisations, including leisure centres and businesses, are reluctant to get rid of their vending machines because they rely on the profits they generate.

“Having to wait for something makes it less desirable,” said Dr Brad Appelhans, who led the research at the Rush University Medical Centre in Chicago.

“Research shows that humans strongly prefer immediate gratification, and this preference influences choices and behaviour in daily life.

“We wanted to see if we could use this preference for immediate gratification to improve people’s vending machine snack choices.”

The research team also found that offering a 25 per cent discount on healthy snacks or imposing a 25 per cent tax on less healthy snacks influenced buying patterns.

Unlike the time delay, however, the second two measures brought in less cash overall.

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Britain is one of Europe's leading users of vending machinesCredit: PA

The Disc (Delays to Improve Snack Choices) system also colour-coded and labeled the healthy snacks from the regular, less healthy options, which were classed as containing 250 calories per serving and more than 25 per cent calories from fat.

Tam Fry, from the National Obesity Forum, said Disc was a “terrific” innovation.

“It ticks all the right boxes and one wonders why no one has thought of this before.

“What's more, its not just a flash-in-the-pan gimmick but it has been field-tested and proven to work.

“To top it all, it appears that vendors can install the system into their standard machines at a viable cost.

“They will have no excuse not to do it."

Earlier this week Public Health England called on sweet manufacturers to cut the amount of sugar in their products by a fifth by 2020, principally in an effort to crack down on childhood obesity.

It is most likely producers will reduce the overall portion size of their sweets and chocolates in order to comply with the new guidance.

The watchdog said it would urge ministers to clamp down on advertising and promotion if the voluntary target is not met.